RF Circulators are three port components used to direct RF energy selectively between the ports as a function of the direction of the RF propagation. Circulators and isolators are typically useful at frequencies ranging from very high frequency (VHF) to microwave frequencies. A typical application involves routing RF signals from a transmitter to an antenna, while blocking undesirable signals reflected back towards the transmitter during a transmission. A circulator does this by routing the reflected signals to a port having a resistive termination to dissipate the reflected energy as heat. When configured this way, the combination of the circulator and the resistive load is called an isolator.
Circulators typically comprise a conductor junction to couple RF energy to the circulator. The conductor is located near a ferrite component situated in a magnetic field, usually provided by a permanent magnet. A passive metal ferrous component completes the static magnetic field caused by the magnet.
Radio signals are coupled to the circulator by transmission lines. Integrated radio circuits generally use integrated transmission lines. The most common types of integrated transmission lines are micro-strips and striplines. Micro-strip lines typically comprise a flat thin rectangular signal-carrying conductor situated above a flat ground plane. Striplines comprise a flat thin rectangular conductor situated between two grounds (planes or slightly larger flat rectangular conductors). In both cases the dimensions of the conductors and the spacing between them establish the electrical characteristics of the transmission line.
FIG. 1 shows an exemplary circulator with stripline transmission lines. Ferrite discs 12 and ground planes 13 surround conductor junction 14 to create the stripline transmission line. Magnets 11 act in conjunction with ferrite discs 12 to form the circulator. FIG. 2 shows an exemplary micro-strip device. Here, conductor junction 14, ferrite disc 12, and ground plane 13 form the micro-strip transmission line. The circulator is formed by ferrite disc 12 operating in the magnetic field established by permanent magnet 11.
Low temperature co-fired ceramic on metal (LTCC-M) is a relatively new packaging technique. It is a superior media because of its high thermal conductivity, good resistivity, and high frequency impedance. LTCC-M devices are mechanically robust, can be hermetically sealed, and are relatively inexpensive to fabricate.
It would be highly desirable to be able to provide RF circulators and isolators with both micro-strip and stripline transmission lines in an integrated LTCC-M package.